Which type of conductor may be directly buried in the ground?

The choice of USE, or Underground Service Entrance cable, is the correct answer because this type of conductor is specifically designed for direct burial applications. USE conductors have insulation that is resistant to moisture and elements typically found in underground environments, ensuring that they can withstand the conditions without degrading over time. This makes them suitable for installations where the cable will be buried in the ground, often used to connect electrical service to buildings in locations where running overhead wires is impractical.

In contrast, the other options listed are not ideal for direct burial. RHW-2, THHN, and XHHW conductors have insulation types that may not provide the necessary protection against moisture and physical damage for long-term underground installation without additional protective measures. These types of conductors are generally intended for use in dry applications or in conduit and may require further consideration for installation in wet or buried conditions. Therefore, USE is the only option that is specifically rated and engineered for direct burial use.